Fix build for systems that have an unsupported version of QT5 installed

This is a proposed solution for this issue:
https://plus.google.com/u/0/102032883547463415401/posts/dFiKM9Nc6Px?cfem=1

The user has Qt 5.3.0 installed. The script detects the installed version and does not use the version installed in his home directory.

I feel that this is a valid solution because we maintain Shotcut in a way that it requires a very specific version of Qt. Alternately, we could check if the supported version of QT is installed in the system path and only use the installed QT if it is the supported QT version.

The reason I haven't seen this problem in Ubuntu is because Ubuntu installs QT 5.2.1with qmake named "qmake" not "qmake-qt5". So the script does not detect the installed version of QT in my ubuntu installation.